Conventional telephone instruments include an incoming call annunciator: classically a bell, more recently a piezoelectric resonator. In a conventional telephone network, a wire pair extends from a telephone central office location to the service subscriber's premises. Central office battery voltage is typically applied to the wire pair to power the telephone service set, and an AC voltage, nominally 90 volts at 20 Hertz, is applied whenever it is appropriate to ring the telephone bell(s) of the service set(s) at the subscriber's premises, in order to signal an incoming call and induce a user to remove the handset from its cradle, known as an "off hook" condition. The standard telephone service set within the U.S. Bell System includes a bell call annunciator which is broadly resonant at 20 Hertz. In Europe and elsewhere, a frequency of 25 Hertz is commonly employed as the ringing frequency.
Ringing voltage is generated at a central office in a number of ways. A rotary generator or ringing machine consists of a single-speed motor (either AC or DC, depending upon the local power supply). The motor rotates one or more AC generators which generate the desired ringing frequencies and voltages. Magnetic generators operating from the AC power mains have been used to generate ringing signals. Such generators employ resistors, transformers and tuned circuits of inductors and capacitors in order to develop the desired ringing signal. Vibrating reed converters have also been employed to generate ringing voltage from the 48 volt central office battery supply. These converters have included two magnetic coils, an armature and a reed assembly mounted on a frame, and have converted the 48 volts DC into a square wave of the desired frequency for ringing. A filter circuit has modified the square wave to a sine wave. A mechanical interrupter has been employed to divide the ringing generator's signal into alternating ringing and silent periods. One implementation has been a motor for rotating a shaft carrying a number of cams which operate switch contacts that switch the ringing signal on and off. Electronic oscillators have also been employed to generate signals used for ringing. Such circuits have been used in conjunction with private branch exchanges, etc. All of the foregoing approaches have been bulky and have required significant amounts of primary operating power.
Fiber optic networks are proliferating within telecommunications systems. Fiber optic cables are now being extended to "curbside" or within about 100 feet or so of the subscriber's premises. In fiber optic based networks, it is not possible to send central office battery or ringing signals over the fiber optic cable, and these signals must be locally generated at the SIU and delivered to the customer in an appearance functionally identical to the existing "wire plant" network capability. While small, switched mode DC to DC converters are known to be efficient in supplying power, a hitherto unsolved need has arisen for a highly compact, highly efficient DC to DC to AC power supply including an on-demand tinging voltage generator, e.g. for inclusion within an SIU of a fiber optic network.